The present invention relates to a permanent-magnetic motor and, more particularly to a permanent-magnetic direct current electric motor for use in an electric driven power steering apparatus such as in an automobile In the present invention, this direct current electric motor comprises a field pole composed of a permanent magnet. The permanent-magnetic motor utilizing the permanent magnet as a field pole is employed as a power steering force assistance means in the electric driven power steering apparatus for use in the automobile.
In the permanent-magnetic motor having permanent magnets as field poles, torque necessary for rotating the motor fluctuates, even when an armature does not receive any electric current supply. This torque fluctuating variation is called cogging torque.
In such a case the cogging torque occurs in the permanent-magnetic motor, and the cogging torque is transmitted to steering in the automobile which is connected to the permanent-magnetic motor, this fact makes an operator of the automobile feel an unnatural force.
The above stated cogging torque results from the difference in the magnetic attraction force in the permanent-magnetic motor. Such a difference in the magnetic attraction force is caused due to the difference in a position relationship between a permanent magnet 102 and a permanent magnet 103 and teeth 125 of an armature iron core 101 as shown in FIGS. 10A, 10B and 10C. A plurality of slots 126 are formed between adjacent teeth 125.
In particular, in case the permanent-magnetic motor has double-pole composed of the permanent magnets 102 and 103 and the number of slots 126 of the motor is an even number, the armature iron core 101 rotates from a first position shown in FIG. 10A and advance to a second position shown in FIG. 10B. The armature iron core 101 rotates further and is made to be positioned at a third position shown in FIG. 10C.
The large torque fluctuation or the cogging torque indicated by a curve line C shown in FIG. 11 occurs in the above stated period during which the armature iron core 101 rotates from the first position to the third position via the second position.
A technique about a reduction for the cogging torque in the permanent-magnetic motor has been developed, for example shown in Japanese Patent Laid-Open No. 30956/1986. This conventional permanent-magnetic motor technique employs a skew method in which the teeth 125 of the armature iron core 101 and the slots 126 between the teeth 125 are skewed as shown in FIG. 7.
In case of the permanent-magnetic motor employing a stew of one slot pitch, for example, the position relationship between an end face portion 114 of the armature iron core 101 at a side of a commutator 117 shown in FIG. 7 and the permanent magnet 102 and the permanent magnet 103 has the first position relationship as shown in FIG. 10A. Then the position relationship between another end face portion 115 of the armature iron core 101 at a side opposite the commutator 117 and the permanent magnet 102 and the permanent magnet 103 has the second position relationship as shown in FIG. 10B. Next, the position relationship between the armature iron core 101 and the permanent magnet 102 and the permanent magnet 103 has the third position relationship as shown in FIG. 10C.
The cogging torque indicated by the curve C in FIG. 11 is generate by a change in the magnetic flux is. The change in the magnetic flux is due to a difference between the magnetic flux when a tooth 125 posses end portions of the permanent magnets 102 and 103 and the magnetic flux when a slot 126 passes end portions of the permanent magnets 102 and 103.
In the above stated permanent-magnetic motor employing a skew of one slot pitch, the rapid change in the magnetic flux due to the rotation of the armature iron core 101 or the torque fluctuation is cancelled out at end face portions 114 and 115 and restrained. Accordingly, the cogging torque generated in the permanent-magnetic motor is reduced.
The cogging torque generated in the permanent-magnetic motor can be reduced effectively by adaption of the skew method with the armature iron core 101, but the effect of the skew method is limited to the portions of the armature iron core 101 which correspond to the lengths of the permanent magnets 102 and 103.
In the design for the permanent-magnetic motor employing the permanent magnets 102 and 103 as the field poles, in general, the effective magnetic flux amount is ensured in accordance with the length at the axial direction of the permanent magnet 102 or 103 is made longer than the end face portions 114 and 115 of the armature iron core 101.
The reason why the length at the axial direction of the permanent magnet 102 or 103 is made longer is that the operating point of the ferrite magnet as the permanent magnet 102 or 103 has low gauss at the 3,000 gauss degree, the permanent-magnetic motor is made compact in size according to the concentrating of the magnetic flux by lengthening the permanent magnets 102 and 103 in the axial direction.
For above stated reasons, it is necessary that the extending portions 124a1 and 124a2 of the permanent magnet 102 and the extending portions 124b1 and 124b2 of the permanent magnet 103 extend beyond the armature iron core 101 to obtain the advantages afforded by lengthening the permanent magnets 102 and 103. Accordingly, an increase in the cogging torque due to the extending portions 124a1 and 124a2 and 124b1 and 124b2 of the permanent magnets 102 and 103, is represented by a line B as shown in FIG. 9. This cogging torque increase is an increase compared with the case where the length of the armature iron core 101 is equal to the lengths, in the axial directions, of the permanent magnets 102 and 103.
Further, there is another technique for reducing the cogging torque in the permanent-magnetic motor, which is called an inequal or non-uniform gap method. In this inequal gap method in the motor, a center line 119 of a radius diameter R of the armature iron core 101 is shifted against a center line 120 of an inner radius diameter R' of the permanent magnet 102, thereby a gap, which is formed between the armature iron core 101 and the permanent magnet 102, is changed.
Also, the center line 119 of the radius of the armature iron core 101 is shifted against a center line 121 of an inner radius diameter R" of the permanent magnetic 103, thereby a gap, which is formed between the armature iron core 101 and the permanent magnet 103, is changed. The inner radius diameter R' of the permanent magnet 102 has the same dimension of the inner radius diameter R" of the permanent magnet 103.
According to this inequal gap method in the permanent-magnetic motor, for example, the change in the magnetic flux amount and the change in the polarity from the field pole of the permanent magnet 102 magnetized at N pole to the field pole of the permanent magnet 103, which is positioned adjacent and has a reverse polarity (S pole) of the former stated field pole becomes moderate. Accordingly, the cogging torque in the permanent-magnetic motor is reduced.
FIG. 13 shows a measurement result of the cogging torque with the skew amount of the armature iron core 101 in the permanent-magnetic motor.
In FIG. 13, a solid curve line D1 indicates the cogging torque in which the permanent-magnetic motor comprises the inequal gap structure according to the prior art. A one-dot chain curve line D2 indicates the cogging torque in which the permanent-magnetic motor has the equal gap structure according to the prior art.
FIG. 14 shows the cogging torque with the eccentric amount between the center line 119 of the armature iron core 101 and the center line 120 of the inner radius diameter R' of the permanent magnet 102 and the center line 121 of the inner radius diameter R" of the permanent magnet 103 in case of the inequal gap method employed in the permanent-magnetic motor.
In FIG. 14, a solid curve line E1 indicates the cogging torque in which the permanent-magnetic motor has a skew of one slot pitch according to the prior art. A one-dot chain curve line E2 indicates the cogging torque in which the permanent-magnetic motor has no skew according to the prior art.
When the length of the armature iron core 101 and the length at the axial direction of the permanent magnet 102 or 103 is made the same, the increase in the cogging torque amount may be avoided. However, with only the combination of the one slot pitch skew method and the inequal gap method, the cogging torque amount cannot be lower than 60 gfcm, because of the size of the permanent-magnetic motor for use in the power steering assistance apparatus in the automobile.
Accordingly, there is a serious problem on the steering feeling sensitivity in which the operator may notice the cogging torque of the permanent-magnetic motor during the power steering operation in the automobile.